Handover (HO) refers to movement of a Mobile Station (MS) from a wireless interface of one Base Station (BS) to a wireless interface of another BS. Hereinafter, a handover procedure in a general IEEE 802.16e system will be described.
A Serving Base Station (SBS) in an IEEE 802.16e network may broadcast information about neighbor BSs through a mobile neighbor advertisement (MOB_NBR-ADV) message in order to inform an MS of information (topology) about basic network configuration.
The MOB_NBR-ADV message includes system information about an SBS and neighbor BSs, for example, a preamble index, a frequency, an HO optimization possibility, Downlink Channel Descriptor (DCD)/Uplink Channel Descriptor (UCD) information, etc.
The DCD/UCD information includes information of which an MS should be aware in order to exchange information through downlink and uplink. For example, the DCD/UCD information includes HO trigger information, and Medium Access Control (MAC) version and Media Independent Handover (MIH) capability information of a BS.
A general MOB_NBR-ADV message includes information about neighbor BSs of an IEEE 802.16e type only. Accordingly, information about neighbor BSs of a type other than IEEE 802.16e may be broadcast to MSs through a Service Identity Information Advertisement (SII-ADV) message. As a result, an MS may acquire information about BSs of a heterogeneous network by requesting a serving BS to transmit the SII-ADV message.
A procedure for an MS having information about neighbor BSs obtained through the above-described method to perform HO in an IEEE 802.16e network will be described in more detail with reference to FIG. 1.
FIG. 1 illustrates an example of an HO procedure which can be performed in an IEEE 802.16e system.
Referring to FIG. 1, an MS exchanges data with an SBS at step SI0I.
The SBS may periodically broadcast information about neighbor BSs to the MS through a MOB_NBR-ADV message at step S102.
The MS may start scanning for candidate HO BSs using an HO trigger condition while communicating with the SBS. The MS may request the SBS to perform an HO procedure by transmitting an HO request (M0B_MSH0-REQ) message when an HO condition is satisfied, for example, when a predetermined hysteresis margin value is exceeded at step S103.
The SBS may inform candidate HO BSs included in the MOB_MSHO-REQ message that the MS has requested HO through an HO request (HO-REQ) message at step S104.
The candidate HO BSs take action for the MS having requested HO to transmit information about HO to the SBS through an HO response (HO-RSP) message at step S105.
The SBS may transmit the information about HO, obtained through the HO-RSP message from the candidate HO BSs, to the MS through an HO response (MOB_BSHO-RSP) message. The MOB_BSHO-RSP message may include information necessary to perform HO, that is, an HO action time, an HO identifier (HO-ID), and a dedicated HO Code Division Multiple Access (CDMA) ranging code at step S106.
The MS determines one Target BS (TBS) among the candidate BSs based on the information included in the MOB-BSHO-RSP message received from the SBS. The MS then transmits a CDMA code to the determined TBS to attempt ranging at step S107.
The TBS receiving the CDMA code may inform the MS of ranging success or failure and physical correction values through a ranging response (RNG-RSP) message at step S108.
Next, the MS may transmit a ranging request (RNG-REQ) message for authentication to the TBS at step S109.
The TBS receiving the RNG-REQ message from the MS may transmit system information, which can be used in a corresponding BS, such as a Connection Identifier (CID) to the MS through a ranging response (RNG-RSP) message at step S110.
If the TBS successfully completes authentication of the MS and transmits all update information, the TBS informs the SBS of the MS of HO success or failure through an HO completion (HO-CMPT) message at step S111.
Next, the MS may exchange data with the TBS which has performed HO at step S112.
The HO procedure for use in the IEEE 802.16m system is similar to the HO procedure for use in the IEEE 802.16e system. However, individual messages may be called in different ways as denoted below.
MOB_NBR-ADV->AAI_NBR-ADV: Corresponding message includes system information transmitted in an S-SFH format rather than a DCD/UCD format.
MSHO-REQ->AAI_HO-REQ
BSHO-RSP->AAI_H0-CMD
RNG-REQ (CDMA code)->Ranging preamble code
RNG-RSP (ranging status) ->AAI_RNG-ACK (ranging status)
RNG-REQ (MAC message)->AAI_RNG-REQ
RNG-RSP->AAI_RNG-RSP: Corresponding message includes a station identifier, such as a TSID or STID, instead of a CID.
In order to reduce an interruption time in an HO process, HO to the TBS may be phased over predetermined intervals while the MS maintains communication with the SBS. This is called Entry-Before-Break (EBB) HO. Although the interruption time can be reduced through EBB HO, the MS cannot receive scheduling from the SBS during an HO execution interval of the MS to the TBS. Accordingly, a method for efficiently performing HO by reducing the HO execution interval of the MS to the TBS is needed.